1. Field of the Invention
The present invention relates generally to engine rpm limiters and, more particularly, to such a limiter which exhibits zero-hysteresis characteristics in that there is substantially no rpm difference between limiting and restarting speeds.
2. Description of the Prior Art
Prior art rpm limiters for internal combustion engines have ranged from mechanical governors to electrical circuitry which usually acts upon the ignition system to prevent the spark plugs from firing and thus lower the engine speed. While some systems act as a simple switch which merely turns off the complete ignition system when an rpm limit is reached and turns it back on when a lower rpm limit is sensed, this produces an rpm difference between the limiting rpm and the restarting rpm, herein defined as hysteresis.
Other, faster operating systems, monitor the time interval between ignition pulses produced when the cam operated breaker switch in the primary of the ignition coil opens and, when the time interval decreases to a predetermined limit, indicative of a limiting rpm, the breaker switch is shorted preventing the generation of the following ignition pulses and, consequently, a spark. Some prior art techniques track or monitor the rpm of the engine by using analog type signal generators which are relatively slow in responding. Thus, when an rpm limit is reached, the breaker switch is usually shorted for a number of ignition pulse times before the rpm signal can again be monitored by means of the ignition pulses. Prior art systems utilizing this technique are illustrated by the patents to Harris, U.S. Pat. No. 3,665,903, Roth, U.S. Pat. No. 3,858,563, and Anderson, U.S. Pat. No. 3,875,915.
Other prior art systems have utilized digital timing techniques for monitoring the time interval between ignition pulses. Such systems are illustrated by the patent to Olson, U.S. Pat. No. 3,738,340. However, when the rpm limit is reached, the breaker switch is again shorted and the following ignition pulse cannot be used for timing. Again, there is a time interval or rpm difference between the limit and restart engine speeds with the result that the engine can cycle between the limit and restart speeds.
Prior art systems which have attempted to preserve the ignition pulses for timing purposes but inhibit the generation of a spark are illustrated by the patents to Kondo, U.S. Pat. No. 3,967,604 and Howard, U.S. Pat. No. 3,704,699. In both cases, the inhibition of the spark is effected by relatively complicated auxiliary circuitry which is a part of the ignition system itself and cannot be simply connected to an existing system.
Therefore, there has long been a need for an rpm limiting system which could monitor the rotational speed of an engine for a predetermined upper limit which would not exhibit the hysteresis effect, and which could be simply attached to an existing ignition system without further modifications. The present invention satisfies that need.